This invention concerns a neutron detector having a current amplifier for the signal which is based on (n,e)-processes. Such detectors, which may have, for instance, cobalt or thulium as the activatable emitter material and are provided with a concentric collector of Inconel, are employed particularly in the so-called "internal core instrumentation" of pressurized-water reactors. Among other things, they serve to monitor the neutron flux density in the core, so that, for instance, in the event of unusual flux changes, a scram action can be initiated for the nuclear reactor. In this context, they therefore constitute an important member of the reactor safety system. From this, it follows that the reliability of such neutron detectors should be monitored.
As the current of a neutron detector proportional to the neutron flux is only fractions of a microampere, each signal must be processed in a current amplifier for subsequent evaluation. The amplifier is connected with the neutron detector, i.e., with its emitter and collector, via cables which lead into the interior of the reactor pressure vessel. These cables are subjected to high temperatures, high pressures and, in addition, to radiation influences. Their insulation resistance can suffer from this, whereby the signal of the neutron detector is falsified.
While it is theoretically possible to ascertain the internal resistance of the cables by means of an ohmmeter, each cable must be disconnected from its detector for this purpose, and connected to the ohmmeter. This results not only in an interruption in the operation of the reactor, but also in generally undesirable costs. In view of the high desired insulation resistance, it is furthermore difficult to ascertain when the small tolerable insulation changes are reached. This invention, therefore, seeks a new approach to ensure the reliability of the measurements obtained with the neutron detectors.